It has become increasingly desirable to improve cooling systems in aerospace applications. Typically, cooling systems provide air conditioning, refrigeration and freezer services, and the like for commercial and other aerospace systems. In general, various known options are available for providing cooling, but such options have drawbacks that limit the design options for aerospace applications.
One known option includes a vapor compression cycle. Vapor compression cycles pass a refrigerant through two-phase operation and can operate efficiently and take advantage of the thermal carrying capacity of a liquid, as opposed to a gas, as well as take advantage of the heat of vaporization of the liquid refrigerant. Thus, through portions of the vapor compression cycle, the cooling system can be much more compact when compared to a gas or air-based system because the fluid being carried is in liquid form. However, vapor compression cycles typically are limited to lower ambient temperature operation and may not provide useful solutions for high ambient temperature operation.
Another known option is a single-phase gas-based system using a gas such as air as the refrigerant. However although air can serve usefully as a refrigerant medium, air is not an efficient thermal fluid, as its heat capacitance is limited to a function of its mass flow rate and heat capacity. Thus, gas-based systems are typically less efficient than vapor compression systems and are typically, for that reason alone, larger than vapor compression systems. Additionally, air systems typically include significant duct passages in order to carry the amount of air that is desired to achieve the amount of cooling typically used for aerospace purposes.
To accommodate the wide range of possible ambient operating conditions of the aircraft, cooling systems for aerospace applications typically use a gas-based system. That is, although it is desirable to reduce mass and bulk in aircraft or aerospace applications, typical cooling systems nevertheless include a more bulky and less efficient gas-based system in order to cover the range of conditions that can be experienced.
In aircraft applications, to reduce system size and weight, trans-critical systems may be employed in which a refrigerant, such as carbon dioxide, transitions through thermodynamic states below and above critical pressures during the refrigeration process. When the refrigerant in such a system passes through a heat exchanger (such as a gas cooler), it rejects heat to a coolant such as RAM air, and the refrigerant remains in the gaseous states during the entire heat rejection process. In spite that the refrigerant remains in the gaseous state at the inlet to the heat exchanger, the refrigerant gas acts more like a vapor, but at the (relatively cooler) outlet, the refrigerant gas acts more like a liquid.
Thus, at the refrigerant inlet the refrigerant is warmer than the cooling air, but as the refrigerant cools temperature of the refrigerant also approaches that of the air, having a temperature difference of approximately zero (even for heat exchangers having an effectiveness below 1). As the refrigerant cools, its specific heat capacity increases significantly, decreasing the temperature difference between the refrigerant and the air. As the refrigerant continues to cool, its specific heat capacity decreases, increasing the temperature difference between the refrigerant and the air. Thus, at the refrigerant inlet and exit, the temperature difference is significant and positive, but in the middle of the heat exchanger the difference tends toward zero. Thus, there is a U-shaped profile of the temperature difference that results due to the change in refrigerant specific heat capacity within the heat exchanger. And, at the point near zero degrees temperature difference (near the middle of the heat exchanger), very little or no heat flux establishes demanding huge heat transfer areas, dimensions and weight of the heat exchangers.
Accordingly, there is a need to improve cooling of the refrigerant with RAM air in trans-critical operation for aircraft applications.